Electrolytic regeneration of spent caustic



Oct. 6, 1953 P. J. GAYLOR ELECTROLYTIC REGENERATION OF SPENT CAUSTIC 3 Sheets-Sheet l Filed Dec. l0, 1949 @S www@ Gil-y@ Oct. 6, 1953 P. J. GAYLOR ELECTROLYTC REGENERATION OF' SPENT CAUSTIC Filed Dec. 1o, 1949 3 Sheets-Sheet 2 INVENTOR.

F. J. GAYLOR ELECTROLYTIC REGENERATION OF SPENT CAUSTIC Oct. 6, 1953 3 Sheets-Sheet 3 Filed Dec. lO, 1949 NM. r E w% IN VEN TOR.

Ilwlwv=== Patented Oct. 6, 1953 ELECTROLYTIC REGENERATION F SPENT CAUSTIC Peter J. Gaylor, Union, N. J., assigner of one-half to Charles W. Rippie, Summit, N. J.

Application December 10, 1949, Serial No. 132,274

12 Claims.

This invention relates to the regeneration of :spent caustic obtained when scrubbing sour gasoline for removal of oxidizable sulfur compounds such as mercaptans, hydrogen sulfide, and the like, More specifically, it deals with an electrolytic process for converting said sulfur compounds to readily removable oxidation products, such as to disuldes, in a simple and effective operation.

The regeneration of spent refinery caustic at the present time is a tedious and costly operation and entails use of large, expensive equipment, including heat exchangers, as well as contamination of the atmosphere. In the removal of mercaptans, the spent caustic is blended with Some oxidation catalyst, such as tannic acid, and the mixture is then blown with air and steam until most of the mercaptans (present as mercaptides) are converted to disuldes, which are skimmed off as an oily layer floating on top of the regenerated caustic. In some cases, the regenerated caustic thus recovered is given an oil wash to complete removal of the disulfides before the caustic is returned to the naphtha scrubbing operation.

In the present process, the spent caustic is merely passed through on electrolytic converter wherein the sulfur compounds such as mercaptans are converted to oxidation products such as disuldes, without the need of air, catalyst, steam, heat exchangers, etc. and since these latter materials are notv employed, there is little, if any, atmospheric contamination involved. Furthermore, by-product hydrogen is produced during the conversion, which may be used in the processing operations, as will be hereinafter set forth.

The invention will be more readily understood by reference to the drawings which illustrate a preferred embodiment of the invention. Figure 1 shows a diagrammatic flowsheet of the processing steps involved, while Figure 2 is a perspective view taken from the rear of one lter press type -of electrolyzer which may be employed for the conversion operation, Front views of the different plates employed in the electrolyzer depicted in Figure 2, are shown in Figures 3, 4 and 5. Similar numerals refer to similar parts in the various figures.

Referring again to the drawings, numeral l represents a scrubbing tower in which sour gasoline (preferably caustic-washed sufficiently to remove most hydrogen sulfide, if present) is fed into the bottom through line 2 while aqueous caustic solution (preferably -20%) is fed into the top through line l24, countercurrent to the gasoline stream. The sweetened gasoline is removed from scrubber l through line 3, while the spent caustic, containing mostly mercaptan impurities, is drawn off the bottom through line 5 and thence through line 6 into surge tank l. Pump 9 draws the caustic from tank 7 and feeds it through line IB into liquid-gas reservoirs I2 and 36 disposed on top of electrolyzer unit I 3. Lines Il and 25 are by-pass lines for the surge tank and pump respectively.

Electrolyzer I3 is of the filter-press type, having anode plates 29 alternately disposed with respect to cathode plates 3|, with insulating diaphragm plates 30 interposed therebetween. Anode and cathode plates 29 and 3 I, respectively, are preferably made of nickel plated iron or graphite and have an outer sealing edge and 63 respectively, and ridged front and back faces 43 and 64, the valleys of the ridges being below the plane of outer sealing surfaces 85 and 63, thereby enabling liquid caustic to travel and migrate therein when the plates are tightly compressed against each other. The intermediate diaphragm plate 30 has a rubber sealing edge surface 55 disposed around the outer edge of the plate on both faces, the surface making a tight seal against surfaces 85 and 63 of the adjacent and oppositely disposed electrode plates. The central portion 56 of the diaphragm plate 30 is made of asbestos or other suitable diaphragm material, enabling liquid to liquid conductive contact therethrough, but resisting diffusion therethrough of gases formed by the electrolysis of the caustic solution.

All of the plates are provided with an opening in each of the four corners. However, some of these openings are sealed off from fiuid within certain of the plates. For example, in anode plate 29 (Fig. 3), the portion of the caustic solution which forms the anolyte in each cell entering through opening 4l flows freely onto the plate surface 43, most of the liquid owing in the valleys between the ridges. During electrolysis, nascent oxygen forms at the anode plate 29, and, if mercaptans are present, this oxygen immediately reacts therewith to produce disulfldes. With proper operation, there should be no excess oxygen produced. However, if some is formed, it will rise in the valleys of the surface of the plate and migrate into top opening 46 and pass through the corresponding (right hand upper) openings in all of the plates, thence through header pipe 39 and into collector tank 35, from which it may be drawn off for any purpose desired.

Electrolyzed anolyte and any excess oxygen flows over surface 43 of plate 29 and passes through opening 46 since sealing edge 50 is not closed off at inner portion 5I, thereby permitting egress of liquid from surface 43 into opening 46 even though all of the sealing edges 59, 58, 69, etc. are in contact with each other as shown in Fig. 2,

Diaphragm plate '3.0115 made ojf tightly woven asbestos cloth, the central portion 56 being permeable to liquid but impermeable to gas (particeularly when wet). Edges 55 `of ,plate 3l) are impregnated with rubber and -lined -with a rubber, Neoprene, or plastic or other suitable film to facilitate a tight seal between plates .and .eliminate leakage of liouid or gas. All of the .cornelithdhings 51, 59, B and 6l are sealed off at the inner portion 50 to prevent any `Aliquid -from entering these openings l, 59, etc. from plate surfaue ,551, although liquid and gas .from other plates can migrate `through .openings .51. 59,. elle..

`Cathode plate ,3l .is constructed s'unilarh7 `to plate .29 `and .is provided with .dat vfront and Joaok sealing .edge .surfaces `lit.. .Hoy/oyen .in the `case of the .corner openings .in plate .3;l the .sealing edges of .the .openings .are .ispose'd .so that 'the portion of `the .caustic solution `which iorms the catholyte .in .each .cell will .now .in .through .the right hand .lower .opening 'L3 (opposite xto the liquid inlet in plate 29).. 'Thiscatholyte .then ows over the valleys, Yloetween the ridges Aon .surface 6d and the regeneratedcaustic .passes out .through diagonally disposed .opening .65., .due to .the sealing .edge .6.6 .being ypartly .open .on inner Vporticm ,51. Hydrogen for-med .during the electrolysis `also ows out, along with .the ,catho'lyta .through the upper left-.handopenings ,65 in allot .the plates.

Accordingly, regenerated .caustic and gaseous electrolytic ,products are ,disohllged .through the upper .corner openings .of ,the plates. the oxygen. if any, collecting, in .this case, in .thorightghint corner openings.. vand gradually Yacouruplating in reservoir 3 5, While hydrogen .passes llihrough .the left .hand :corner .openings ,and .eventually collects in reservoir .I2 from which it may ,be drawn ...for any use required therefrom.

Regenerated .caust' leaving electrolyzer 1 3. through 4Vvalve ,IB Yand line .L1 is run .or .is .Dumped into settling .tank .I-.8 wherein.aqueousrogenerated caustic ..20 settles `out .at .the disuldes .form ,an oily layer .L9 which ,may 'loe drawn loutof .tank I8 through .line '11.. The .regenerated .caustic 2. .may then .be drawn `,out of tank I8 through line 2| and A,munped b y pump 22 .through line .2.3 .back into oaustic .line 2d in the ,gasoline .scrubbing operation. 'resh caustic, in .small .replenishing amounts, may be .fed .in through line ..4 Qr lille .30.-

Hydrogen `formed the .electrolysis may "be collected in chamber I2 and `drawn ,o i through line 19 for yuse .in .any of the `.desired renery operations.. Y

AAlthough the hriver filter press-type .electrolyzer has been described :for reflecting roollueriol of .the amercaptans to Ldisuldes, `other :types -zof electrolyzers Ymay 'be used, :particularly those adapted -to produce hydrogen :and oxygen, .and therefore 4provided with 'permeable .diaphragms between the electrodes, ,since it is essential to keep the hydrogen and .oxygen separated. The term mercaptan" Yemployed'herein also includes mercaptides, Ain which form the mercaptans are present .when reacted with caustic. A150.. .eil-Y though caustic soda is specied in the examples` bottom., Vwhile the set forth, it is understood that potassium hydroxide is not precluded as the treating agent.

Regenerated caustic, containing the oxidized sulfur compounds (in this case disuldes) passes through the upper corner openings and is collected and drawn off through valve I6 and line Il. As can be deduced from Figure 2, rear header 26 :which ,also .comprises ,a fportiqn of :the frame of v`the dlter-pregss type zunit, Viis provided with upper channels for facilitating collection of the ygases passing through corner openings, such as channel 85 for collecting hydrogen from upper ileflthandopeningsil'. In this case, a leader pipe 86;is inserted in reservoir l2, connecting tightly Wltllrchahheli .to cprevent any leakage or seepage-ofsperrt paustic 91 coming in through line 35. Liquid 91 acts -as a seal for the hydrogen collected -in `reservoir ll2.

,imilarly, lower channels are provided in rear header 26 to allow flow of spent caustic into the lower Lcorner .openingsof the plates. In Fig. 2,

Channel .81 .is .shown .for .feeding `-the .lower Aleft hand openings in Corners .liil .of .the plates.

LEZrQnt .header 32 .the .member .against .which pressure is aupliedhyihand Wheel .3.3 which turns screw oo st and thusimposes a thrust on header 3.2.. squeezing the plates between .both headers. Front header '3;2 if. provided with channel .a4 .for draining regenerated liquid fromhoth upper .corners of the plates .and .leading saidliduid `through valve l6 .andjline '11 to .outside 'the electrolyzen Lues 62 .are provided .on the .sidesof all .plates and these .rest on insulated .shafts 5 21 which .are Supported by .supports 14.2. lPositive electrical leads T5 .on the positive plates are connected with the positiveline .lead Sli, while the negative leads 3.8 .on the negative plates l.are led to main negartive .electrode .'I4l.,pot.h.r.r 1ai.n .electrodes being l.fed direct .Current by .a converter, .motor .generator set. or the like.. 'Ihe .electrical connections lll may .b emounted .on insulating ,panel du as shown in .Figure .2..

.Instead of .separating the 'hydrogen .from .the regenerated liquid. Iit is also ,possible tto eliminate reservoir l2 -(and 36 when no free oxygen is made.) and .discharge the hydrogen .and .regeneratedliduid 'through openings 157i (Fig. 2,),and Channel .84, out .into 'tank .l8 (Fig. .1) .where .the hydrogen ymay he .collected .in .the .top of tank I8 .(Fig. .1.) .and drawn .off .through line .8.2 when desired.

..Disulfides .and other insoluble .oxidation products, .which are .torinesi .by the oleotrolytic oxidation..a.re.al1owod .to oollectin tank Vlit .as an upper layer L9 ,and drawn ,off through limeli.. Nahhtha or kerosene may he runinto Vtank LIB as .a .spray through line .8l to more efciently .remore .oily oxidation ,products from `the regenerated caustic. It ispossible .to .make tank .L8 in .the .form .of .a scrubbing tower .toohtain .better .contacting with naphtha .from line .8.1. Y

The prescntmethod is .also .effective in regenerating spent .caustic .containing hydrogen `sulfide impurity. .Oneadyantageous eature .of Asuch a 1process .is 4the vfact that .a considerable ,portion of the 'hydrogen sulde .is oonrerted .to insoluble sulfur whiohmay loefiltered .oli from the Aregenerated caustic leaving Vthe electrolyzer, o r collected as sediment in tank I8.. Although electrolyzer |,3jheats up somewhat during -theccnversion operation, .the heat is not excessive, .although thev application of cooling .of the pla-tes is not fpreeluded.

of over 0.1 ampere per square inch of projected area are preferred. The term projected area relates to the overall dimensions of the plate and does not take into consideration the additional surface provided by the ridges on the plate surfaces.

The following' examples are submitted for the purpose or illustrating, but not in any manner limiting, the invention heretofore outlined:

Eample I An aqueous 20% solution of sodium hydroxide containing 0.368% mercaptan by weight was eleotrolyzed in a Shriver type electrolyzer, the plates of which measured 7" x 7". A voltage of 6 volts and amperage of '7 amps, was imposed on the solution for 10 minutes, with a mild iiow of caustic flowing between the plates. The regenerated caustic withdrawn from the electrolyzer, and separated from the disulde produced., analyzed 0.071% mercaptan by weight, showing a substantial removal of the oxidizable sulfur from the spent caustic.

Eample II In the same type of unit as that described in Example I, a 20% aqueous caustic soda solution containing l0.68% mercaptan sulfur (as butyl mercaptan) was electrolyzed similarly for l0 minutes with 3 volts and 2 amps. rlhe mercaptan sulfur on the separated regenerated caustic analyzed 0.57%. After 20 minutes, the mercaptan sulfur analyzed 0.46%, after 35 minutes, 0.35% and after 40 minutes 0.24%, showing that the oxidation of the mercaptans is almost directly proportional to the electrical energy supplied.

Although plain caustic containing oxidizable sulfur compounds which are oxidizable to readily removable or separable products has been disclosed, it is also possible to employ spent caustic containing solutizer reagents, tannic acid, alcholates, phenates, cresylates, naphthenates, isobutyrates, and the like.

One advantage in eliminating reservoir l2 and allowing the hydrogen to collect in tank I8 with the regenerated caustic is that much better control of liquid fiow may be obtained over the electrode plates. Furthermore, it is possible (by changing the size oi the corner openings, or by employing pressure) to obtain a greater flow of caustic over the cathode or anode plates (as desired), to eiect a higher overall efciency in the conversion operation.

Although a caustic strength of -20% is mentioned herein, it is to be understood that other strengths may be employed, particularly more highly diluted solutions, say of 1% to 10%, as an example. The present process would allow such aqueous caustic containing hydrogen sulde, in free and/or combined form, to be converted in high yields to free sulfur which would be present as a dispersion in the liquid stream leaving the electrolyzer, and which may be removed by settling and/or ltration of said liquid stream.

I claim:

l. The process of regenerating spent aqueous caustic solution containing oxidizable sulphurcontaining impurities which comprises passing a stream of the caustic soiution through a diaphragm-type electrclytic cell having insoluble electrodes, impressing on said electrodes a current density oi such intensity as to liberate nascent oxygen in direct contact with the oxidizable sulphur-containing impurities in the anolyte, and maintaining said solution in the cell,

least a substantial portion of the sulphur-containing impurities thereof.

2. The process of regenerating spent aqueous caustic solutions as deiined in claim 1 in which the sulphur-containing compounds are mercaptides and the oxidation products are disulphides, and which includes withdrawing the treated caustic solution from said cell, and separating the disulphides from the withdrawn treated solution.

3. ylhe process of regenerating spent aqueous caustic solutions as defined in claim 2 in which the oxidizable sulphur-containing compounds include hydrogen sulphide and the product of oXidation thereof is sulphur, and which includes withdrawing the treated caustic solution from said cell, and separating the sulphur from the withdrawn treated caustic solution by filtration.-

4. The process of regenerating spent aqueous caustic solutions as defined in claim 1 in which the surface of the electrodes exposed tc said solution are nickel.

5. The process or" regenerating spent aqueous caustic solutions as dened in claim 1 in which the surface of said electrodes exposed to said solution are graphite.

6. The process cf regenerating spent aqueous uaustic solutions as defined in claim 1 in which the current density is so correlated with the amount of oxidizable sulphur-containing impurities in the caustic solution and the time the solution remains in said cell that substantially all of the sulphur impurities are oxidized without substantial liberation of free oxygen.

'7. The process of regenerating spent aqueous caustic solutions as dened in claim 1 in which the current density impressed on the electrodes is at least 0.1 ampere per square inch of projected area.

8. The process of regenerating spent aqueous caustic solutions as defined in claim 1 which includes liberating hydrogen at the cathode and removing the liberated hydrogen along with catholyte.

9. The process of regenerating spent aqueous caustic solutions as dened in claim 8 in which the current density is so correlated with the amount of oxidizable sulphur-containing impurities in the caustic solution and the time the anolyte remains in contact with the anodes that a small amount of free oxygen is liberated, and which includes removing the excess oxygen from the cell along with anolyte.

10. The process of regenerating spent aqueous caustic solutions as dened in claim 9 which includes separating the hydrogen and oxygen, respectively, from the anolyte and catholyte portions of the caustic solution and returning the caustic solution to said cell for further treatment.

11. 'I'he process of regenerating spent aqueous caustic solution containing oxidizable sulphurcontaining impurities which comprises passing a stream of the caustic solution in a closed system through a plurality of diaphragm-type electrolytic cells having insoluble electrodes, impressing on said electrodes a current density of such intensity as to liberate nascent oxygen in direct contact with the oxidizable sulphur-containing impurities in the anolyte of each cell, and maintaining said solution in said cells, while such electrolysis continues, for a length of time suicient tial portion orv the sulphur-containing impurities thereof.

l2. ,process 'tlhvllA @Qmprses bringing hydrocarbons containing oxidizable sulphur-containing impurities into intimate contact with an aqueous caustic solution, separating from the hydrocarbon the aqueous caustic solution including sulphur-containing impurities taken up from the hydrocarbon, vpassing a stream of the caustic solution including the ioxidizable sulphurcontaining impurities througha plurality of diaphragm-type electrolytic cells having insoluble electrodes, impressing on said electrodes a current density of such intensity as to liberate nascent oxygen in direct Contact with the oxidizable sulphur-containing impurities in the anolyte o each cell, maintaining said solution in lsaid cell,v

while said electrolysis cont'nues,

for a length of time suicient to bring about oxidation of at least a substantial portion of the sulphur-containing impurities of said solution, withdrawing the 8 treated, caustic. solution, Separating the oxidized sulphur-@Gambling impurities from the withdrawn treatedl solution, and returning `the caus-v tic l'solution fromWhichi'fhe'oxidized sulphur-containing impurities were Aseparated for the treatment of further amounts |of hydrocarbon.

PETER J. GAYLOR.

'References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,420,212 Paulus June 20, 1922 2,140,194 Yabroff et al. Dec. 13, 1938 FOREIGN EVA'IYENTS Number Country. Date 8.00 Great' Britain :'1=f:" of 15885 OTHER REFERENCES Electro-organic Chemistry, by Broekman,

published in 1926, pages 5 and 6. 

1. THE PROCESS OF REGENERATING SPENT AQUEOUS CAUSTIC SOLUTION CONTAINING OXIDIZABLE SUPLHURCONTAINING IMPURITIES WHICH COMPRISES PASSING A STREAM OF THE CAUSTIC SOLUTION THROUGH A DIAPHRAGM-TYPE ELECTROLYTIC CELL HAVING INSOLUBLE ELECTRODES, IMPRESSING ON SAID ELECTRODES A CURRENT DENSITY OF SUCH INTENSITY AS TO LIBERATE NASCENT OXYGEN IN DIRECT CONTACT WITH THE OXIDIZABLE SULPHUR-CONTAINING IMPURITIES IN THE ANOLYTE, AND MAINTAINING SAID SOLUTIONIN THE CELL, WHILE SUCH ELECTROLYSIS CONTINUES, FOR A LENGTH OF TIME SUFFICIENT TO BRING ABOUT OXIDIZATION OF AT LEAST A SUBSTANTIAL PORTION OF THE SULPHUR CONTAINING IMPURITIES THEREOF. 